Can’t you make a definitive diagnosis from an autopsy. Could be that multiple diseases lead to the same findings in the autopsy.

While someone is alive a diagnosis of Alzheimers is more of a diagnosis of elimination. There are several drugs that can be used to slow the progression and I assume that those have a role to play in solidifying the diagnosis.

You can check for amyloid plaques in an autopsy, but apparently plenty of old people that show no significant cognitive decline also have amyloid plaques.

So, while we would definitively call it Alzheimer's if you have significant cognitive decline + amyloid plaques, it's not 100% clear that this is a single diagnostic.